Immigration is at the center of the 2024 election for many. Why?

President Joe Biden and former President Donald Trump are both visiting the southern border on Thursday, with the issue of the border and high rates of illegal immigration as a flashpoint between them in the race for the White House.

The Biden campaign slammed Trump in a statement ahead of his border visit for "playing games for his own political gain," a spokesperson said, after Trump told Senate Republicans not to support a bipartisan border security deal, which Trump has criticized as ineffective.

A Trump campaign spokeswoman, meanwhile, said in her own statement that Trump is visiting "the crime scene of Biden's open border … he will outline his plan to put America first and secure the border immediately upon taking office."

Here's how Americans feel about the issues of immigration and the border and about which front-runner candidate would handle it better, according to some recent polling.

How important is immigration and the border to Americans?

According to February polling from Gallup, more American adults thought immigration is the most important problem facing the country than other issues -- but that concern was driven mostly by Republicans.

Gallup found that 28% of Americans overall said immigration is the United States' most important problem, up from 20% in January and 9% last August in their polling.

But there has been little movement among Democrats, only 10% of whom cited immigration as the most important problem in February and 9% in January.

By contrast, 57% of Republicans did, up from 37% in Gallup's January polling.

Independents were in the middle: 22% said immigration was the most important problem facing the nation in February, up from 16% in January.

A separate poll by Quinnipiac University released in February found that immigration was the third-ranked most important problem among registered voters -- with 17% of those polled saying it was the most urgent issue -- behind "preserving democracy in the United States" (21% of registered voters) and the economy (20% of registered voters).

When broken down by party, Republicans were more likely than Democrats to say immigration was their choice of top issue. Among Republicans, 35% called it their most urgent issue but only 4% of Democrats did the same, while 16% of independents felt that way.

Biden's handling of immigration and the U.S.-Mexico border

Biden's job approval ratings on immigration issues are overall relatively low, although an analysis of polls by 538 found that his average approval rating on immigration and the border has ticked down by only about 5 points since spring 2023.

According to the most recent polling on Biden's job approval on these issues conducted by ABC News/Ipsos, from mid-January, his rating for handling immigration at the U.S.-Mexico border was especially low -- just 18% approved, about half what it was in spring 2021, while 63% disapproved.

Biden also has the lowest rating on immigration for any president in past ABC News/Washington Post polls to ask the question since January 2004 (with various changes over time in question wording).

Gallup's polling in February found that among American adults, 28% approved of how Biden is currently handling immigration, while 67% disapproved. This was largely unchanged from Gallup's August 2023 polling on the issue, which found that 31% of Americans approved of Biden's handling and 66% disapproved.

Among the Republican adults Gallup polled in February, 97% said they disapproved of Biden's handling of immigration, while 37% of Democratic adults said the same. Fifty-five percent of Democrats told Gallup they approved of how Biden has handled the issue, while only 3% of Republicans felt that way.

Quinnipiac University's poll in February found similar trends: 29% of registered voters said they approved of how Biden "is handling the situation at the Mexican border," while 63% said they disapproved.

Among the Republican voters polled, 93% disapproved of Biden's handling of the issue, while 31% of Democrats said the same. Over a majority of Democrats, 58%, said they approved of how Biden has handled the issue, while only 4% of Republicans affirmed that way.

Biden versus Trump on the border

When Biden and Trump are pitted against each other in some polls on issues of immigration, Americans overall indicate they feel Trump is handling the issues better, with some polls again showing that Republicans overwhelmingly back Trump on the issue.

A poll from ABC News/Ipsos in February found that 26% of American adults trusted Biden to do a better job handling immigration and the situation at the U.S.-Mexico border, while 44% trusted Trump more.

About the same number of Americans also indicated they blamed Republicans in Congress (53%), Democrats (51%) and Biden (49%) on Congress' failure to pass legislation intended to decrease the number of illegal crossings at the U.S.-Mexico border while 39% of those polled blamed Trump.

According to a February poll from Marquette University Law School, when asked who was better on immigration and border security, 53% of registered voters said Trump was handling it better while 25% said Biden was.

A sizable number of registered voters polled by Marquette equivocated: 15% said neither was good with the issue, and 6% said they were both the same.

Once again, the approval for each candidate broke down largely among party lines. In Marquette University's poll, 92% of Republicans or Republican-leaning independents polled said Trump would do better, while 51% of Democrats or Democratic-leaning independents said Biden would.

Twenty-five percent of those Democrats polled said neither would be good with dealing with the issue, and 3% of Republicans said the same.

ABC News' Christine Filer, Gary Langer, Isabella Murray and Nathaniel Rakich contributed to this report.